Liposomes composed of phosphatidylethanolamine and oleic acid (PE/OA) become unstable and fusion-active at the weakly acidic pH of from about 5 to about 6.5. These pH-sensitive liposomes can be coated with fatty acid-derivatized antibody to enhance the cytoplasmic delivery of encapsulated molecules to antigen expressing cells.
Cytoplasmic delivery is thought to be achieved through receptor-mediated endocytosis of the immunoliposomes. The liposomes then encounter the acidic pH of the endosome and are thought to fuse with the endosome membrane, thus releasing the encapsulated contents into the cytoplasm.